Sure a very good question.The retinoid, the chemical of accutane binds with certain receptors in the developing foetus, like the mesoderm ,effecting the terminal differentiation of cells leading to malformations.The effect on ectoderm ie skin is totally different.This difference makes the reason for different approaches which you have very rightly spotted.
Thats a really good question...and I would be interested in an answer from a medical professional too! My guess is that Accutane permanently changes your skin down to the chemical level, making it very sensitive to harsh exfoliants for months following treatment. However, it doesn't stay in your blood stream for that long after terminating treatment and that is what would harm an unborn child. Does that make sense?